Heat-curable elastomeric silicone compositions

ABSTRACT

A heat curable elastomeric silicone composition comprising a mixture of two polydiorganosiloxanes, each of which contains vinyl unsaturation; a silica filler; a polyorganohydrogensiloxane and a platinum compound; and a method for the preparation of vulcanizates thereof. The composition is cured for a period up to about one hour and is then post-cured for up to about 24 hours to yield a vulcanized elastomer having high tear strength, flame retardance, heat resistance and superior compression set.

United States Patent [451 June 20, 1972 Wada et al.

[54] HEAT-CURABLE ELASTOMERIC SILICONE COMPOSITIONS [72] Inventors: TM Weds: Kunlo Itnlt, both of Annalta-shi. Japan [73] Assignee: Sltlnetnu Chemical Company. Tokyo,

Japan |22| lilcd: Dee. Ill. I969 [2]] Appl. No.: 886,372

[30] Foreign Applcnllon Priority DI- Dec. 23, I968 Japan "43194385 [52] US. Cl. ..Z60/29.l SI, 260/37 SB [5]] Int. Cl "C0835!!!" [58] Field Search ..260I37 88, 46.5 U. 825, 29.l G

3.220.972 11/1965 Lamoreaux......................,...260/46.5 3,284,406 "966 Nelson ...260/46.5 134L490 9/l967 Burdiclt et al.... .260/37 SB 3.436.366 4/l969 Modic ..260l37 SB Primal v liruminer-Lewis T. Jacobs AIMHMT-MLG|W and 'lurcn I 57) ABSTRACT compression set.

8 Claims, No Drawings HEAT-CURABLE ELASIOMFJUC SILICONE COMPOSITIONS BACKGROUND OF THE INVENTION This invention relates to novel elastomeric silicone cornpositions, and particularly to heat-curable elastorneric silicone compositions possessing greatly improved tear strength.

Prior art silicone rubbers have been prepared by curing with a peroxide such as benzoyl perioxide, tertiary butyl peroxide, dichlorobenzoyl peroxide and tertiary butyl perbenzoate, compositions consisting of l. polydiorganosiloxane gum in which the organic groups are mostly methyl groups with a small number of vinyl B -P 2. siloxane ester of low molecular weight or silanol, and

3. a filler.

As these vulcanizates are superior in electrical properties and thermal and weather resistances. they are used in various applications, but they have a big disadvantage in that they have poorer tear strengths than synthetic rubbers other than silicone rubbers. Various attempts have been made to overcome this disadvantage. In one case, some of the organic groups in the above-described polydiorganosiloxane gum were replaced by phenyl groups which served to improve the tear strength of the product to some extent, but which caused the elasticity to be reduced in addition to causing a remarkable degradation of the compression set and the oil resistance thereof. In another case, although the silica filler employed for the purpose of reinforcing had a surface area of at least 300 mlg, the tear strength of the product proved to be no more than 20 ltglcm. Thus the problem of how to prepare silicone rubbers having higher tear strengths has not before now been solved.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide heatcurable elastomeric silicone compositiom which have high tear strengths.

Another object of this invention is to provide heat-curable elastomeric silicone compositions which, when cured, give silicone rubbers which are superior in compression set, heat and steam resistances and which are self-extinguishing in addition to having improved confined heat resistance, and which contain therein no decomposed residue of the curing agent.

Yet another object of this invention is to provide heat curable elastomeric silicone compositions which are useful as wireinsulating materials, sealing materials, and packing materials.

Still another object of this invention is to provide a process for obtaining vulcanizates of the above elastomeric compositions.

The compositions of the invention comprise:

a. from about 70 to about 99.5 parts by weight of a polydiorganosiloxane containing from about 0.02 to about 0.2 mole percent of vinyl group-containing siloxane units and which is represented by the general formula: R,Si(R,Si0),,,SiR

wherein R is the same or different monovalent hydrocarbyl group, either substituted or unsubstituted, and m is at least 3,000;

b. from about 0.5 to about 30 parts by weight of a polydiorganosiloxane containing at least about 2 mole percent of vinyl groupcontaining siloxane units and which is represented by the general formula:

where R is the same or different monovalent hydrocarbyl group, either substituted or unsubstituted, and n is at least I00;

c. from about 20 to about 200 parts by weight of a silica filler having a surface area of at least 150 m'lg;

d. a polyorganohydrogensiloxane in an amount such that its Si-H groups are from about 80 to about 250 mole percent relative to the total vinyl groups contained in the above components (a) and (b), and which have at least 3 Si-H groups in each molecule; and

e. a platinum compound.

The above composition is cured for a period up to about one hour and is then post-cured for up to about 24 hours to yield a vulcanized elastomer having high tear strengths, flame retardance, heat resistance, and superior compression set.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention has been made through our observations that when polyorganosiloxane compositions formed of silicone rubbers consisting of the above components (a) and (b) are heated either under atmospheric pressure or high pressure in the presence of a curing agent of polyorganohydrogensiloxane, and a platinum compound, silicone rubber vulcanizates with an unexpectedly high tear strength can be prepared.

To give a more detailed description of the components employed in practicing the instant invention, polydiorganosiloxane employed as component (a), is represented by the formu- R.SiO(R,SiO)..SiR, where R is the same or different monovalent hydrocarbyl group, either substituted or unsubstituted, for example, vinyl, methyl, ethyl, trifluoropropyl, or phenyl group. Said polydiorganosiloxane is obtained by polymerization in the presence of an alkaline catalyst, for example, a mixture of octamethyl tetracyclosiloxane and tetrarnethyl tetravinyl cyclosiloxane, and, if desired, octaphenyl tetracylosiloxane or tetramethyl tetraphenyl cyclosiloxane. This is the principal component of the silicone elastomeric composition, so it must have a degree of polymerization of at least 3,000 in order to prevent the reduction of the tear strength of the silicone rubber vulcanizate prepared therefrom.

The polydiorganosiloxane which is employed as compound (b) and which is represented by the general formula:

R,Si0( R',Si0),,SiR', where R is the same or different monovalent hydrocarbyl group, either substituted or unsubstituted, for example, vinyl, methyl, ethyl, trifluoropropyl, or phenyl group, is prepared just like component (a), but its degree of polymerization does not have to be as high as compound (a), although it has to be at least 100.

Both components (a) and (b) must contain vinyl groups, but in order to obtain silicone rubber vulcanizates having a high tear strength, the vinyl group-containing siloxane units therein must be from about 0.02 to about 0.2 mole percent in component (a) and at least about 2 mole percent in component (b). The ratio of component (a) to component (b) has to be from about 70 to about 99.5 parts by weight of (a) to from about 0.5 to about 30 parts by weight of (b). If the vinyl group-containing siloxane units in components (a) and (b) and/or the ratio of component (a) to component (b) are not in the ranges given above, the crom-linking reaction with polyorganohydrogensiloxane which is employed as component (d) will not be conducted satisfactorily, failing to give the desired high tear strength to the resultant silicone rubber vulcanirate.

Ordinary silica fillers used for prior art silicone rubbers can be employed as component (c), and can be exemplified by finely divided powders of silica such as fumed silica and silica aerogel. If the particle size of the silica is large, the mechanical strength of the silicone rubber vulcanizate obtained will be reduced, so that the particular size of the silica must be fine enough to have a surface area of at least I50 mlg as measured by the nitrogen absorption method. The quantity of such silica to be incorporated in the composition is from about 20 to about 200 parts by weight relative to the above quantities of components (a) and (b).

The polyorganohydrogensiloxane which is employed as component (d) must have at least 3 Si-H groups in each molecule and is exemplified by trialkylsilyl end-blocked polymethylhydrogensiloxane of various degrees of polymerization, tetrasiloxane represented by Si I K sh L. siloxane copolymer consisting of SiO, units and units and copolymers of methylhydrogensiloxane and dialky1 siloxane. These may be prepared by the known methods. The cross-linking reaction of component (d) is conducted by the addition reaction between Si-H groups and vinyl groups connected to silicon atoms in the above components (a) and (b), so it is preferable that the organic group directly bonded to the silicon atom connected to a hydrogen atom in component (d) is low in molecular weight, e.g., a methyl group.

If the quantity of component (d) is too small, insufficient cross-linking will occur, but if it is too large, the desired tear strength and the thermal resistance of the product will be reduced. Accordingly, it is desirable that the Si-H groups in component (d) be from about 80 to about 250 mole percent of the total vinyl groups contained in components (a) and (b).

The platinum compound added to the composition as component (e) is employed as a catalyst for the above curing, or addition, reaction, and is preferably a soluble platinum compound such as chloroplatinic acid, or a complex of (1) platinum chloride and (2) an olefine such as ethylene, propylene, butadiene and cyclohexene. The quantity of compound (e) required is from about 0.5 to about 20 ppm based on the total weight of the above-mentioned components (a), (b) and (d The heat curable elastomeric silicone compositions of this invention are prepared by kneading the mixtures of the above components (a), (b), (c), (d), and (e) by means of a two-roll rubber mill or a Banbury mixer together with a dispersing agent such as a siloxane ester of low molecular weight or silanol, e.g., diphenylsilanediol, a heat-resistance improving agent such as ferrous oxide, cerium oxide, or ferrous octoate, and pigments. in order to further improve the stability in storage of the products, organic phosphorus compounds or or ganic amines known to the an may be added. There is no particular order in which the above components need to be mixed, but usually components (a), (b), and (c) are homogenously mixed first, and then to this mixture are added components (d) and (e).

The compositions of the present invention are heated at a temperature between about I" C and about 400 C, either under atmospheric pressure or high pressure, for a period of from about 30 seconds to about 1 hour, and then, ifdesired, at a temperature between about 150 C and about 250 C for a period of from about 2 to about 24 hours to accomplish postcuring, thereby obtaining elastomer vulcanizates possessing high tear strength. No residue of the curing agents remains in the finished products unlike the case of ordinary silicone rubbers for which peroxides are usually used as curing agents, so that silicone rubber vulcanizates thus prepared are superior in compression set, heat and steam resistances, and flame retardance a well as confined heat resistance. Consequently, they are very useful as wire-insulating materials, sealing materials, and packing materials especially in such places where high tear strengths are requiredv The details of the invention will be further described in the following examples in which all parts and percents are by weight.

EXAMPLE 1 Trimethylsilyl end-blocked polydiorganosiloxane (component 0) having a degree of polymerization of about 8,000 and consisting of about 99.85 mole percent of dimethylsiloxy units and 0.l5 mole percent of methyl vinyl siloxy units; trimethylsilyl end-blocked polydiorganosiloxane (component blll) having a degree of polymerization of about 5,000 and consisting of 97 mole percent of dimethylsiloxy units and 3 mole percent of methyl vinyl siloxy units or trimethylsilyl endblocked polydiorganosiloxane (component b[ll]) having a degree of polymerization of about 8,000 and consisting of 99 mole percent of dimethylsiloxy units and 1 mole percent of methyl vinyl siloxy units; fumed silica (component c) having a surface area of 200 m'lg; and polymethylhydrogensiloxane (component d) containing units and SiO, units in the ratio of 2 moles to 1 mole; and chloroplatinic acid (component e) were compounded in the ratios given in the following table. First, components (a), (b), and (c) were kneaded on a two-roll rubber mill together with 5 parts of diphenylsilanediol, and after each kneaded composition was heated at 150 C for 1 hour, components (d) and (e) were added. The mixtures were pressed at 170 C under a pressure of 30 kg/cm for 10 minutes into a 2 mm thick sheet. When these sheets were subjected to post-cure heating at 200 C for 4 hours, their elastomeric properties were as shown in the table. The figures in the parentheses are the values obtained of sample sheets that had not been subjected to postcure heating at 200C for 4 hours.

In order to test the heat resistance, two sample sheets of the vulcanizate of Example I which had been post-cured were heated, one at 200 C for 72 hours and the other at 250 C for 24 hours, alter which the tear strengths were 33 kg/cm and 29 kg/cm, respectively. The sheet prepared in Control 2, when subjected to heating at 250 C for 24 hours was found to have a tear strength of no more than 18 kgjcm.

EXAMPLE ll 70 parts of dimethyl vinyl silyl end-blocked polydiorganosiloxane (component a) having a degree of polymerization of about 6,000 and consisting of 99.9 mole percent of dimethyl siloxy units and 0.10 percent of methyl vinyl siloxy units; 30 parts of dimethyl vinyl silyl end-blocked polydiorganosiloxane (component b) having a degree of polymeriza tion of about 6,000 and consisting of mole percent of dimethyl siloxy units and 5 mole percent of methyl vinyl siloxy units; 40 parts of fumed silica (component c) having a surface area of 380 mlg; and 7 parts of diphenylsilanediol were compounded on a two-roll rubber mill and heated at C for 3 hours. Subsequently, 2.2 parts of polydihydrogensiloxane (component d) and 7.5 ppm of chloroplatinic acid (component e), as in Example I, were added to the above composition and were uniformly mixed therein. The composition was then pressed at C under a presure of 50 kg/cm for 15 minutes into a 2 mm thick sheet. The sheet was subjected to heating at 200 C for 4 hours to give a vulcanirate having the following elastomeric properties:

Hardness: 52 Elongation (M 580 Tensile strength (kg/cm) 100 Tear strength (kycm) 48.

Two sample sheets of the above were heated, one at 200 C for 72 hours and the other at 250 C for 24 hours, and their tear strengths were found to be 39 kg/cm and 32 kg/cm, respectively.

EXAMPLE ill 99 parts of dimethyl vinyl silyl end-blocked polydiorganosiloxane (component 0), having a degree of polymerization of about 8,000 and consisting of about 96.6 mole percent of dimethyl siloxy units, 3.3 mole percent of diphenyl siloxy units and Ill mole percent of methyl vinyl siloxy units; 1 part of dimethyl vinyl silyl end-blocked polydiorganosiloxane (component b) having a degree of polymerization of about 1,000 and consisting of 80 mole percent of dimethyl siloxy units and 20 mole percent of methyl vinyl siloxy units; 45 parts of fumed silica (component having a surface area of 300 m'lg; and parts of diphenylmethylsilanemonool were compounded on a two-roll rubber mill and heated at 200 C for 2 hours. Subsequently, 1.05 parts of polydihydrogensiloxane (component d) and ppm of chloroplatinic acid (component e), as in Example I, were homogenously dispersed in the above composition which was then pressed into a 2 mm thick sheet. The sheet was cured with hot air (300 C) under atmospheric pressure and then subjected to another curing cycle at 200 C for 2 hours. The product was found to have the following elastomeric properties in addition to a particularly superior flame-extinguishing property:

Hardness $2 Elongation (9b) 620 Tensile strength (kg/cm) I03 Tear strength (kycm) 50.

When two samples of the above sheet were heated at 200 C for 72 hours and at 250 C for 24 hours, respectively, their respective tear strengths were 40 kg/cm and 32 kg/cm, confirming that they had excellent heat resistance.

EXAMPLE IV 99 parts of dimethyl vinyl silyl end-blocked polydiorganosiloxane (component a) having a degree of polymerization of about 10,000 and consisting of 99.85 mole percent of dimethyl siloxy units and 0.l5 mole percent of methyl vinyl siloxy units; one part of dimethyl vinyl silyl end-blocked polydiorganosiloxane (component b) having a degree of polymerization of about 200 and consisting of 50 mole percent of dimethyl siloxy units and 50 mole percent of methyl vinyl siloxy units; 50 parts of fumed silica (component c) having a surface area of 300 mlg; and 5 parts of diphenylsilanediol were compounded on a two-roll rubber mill and heated at I60 C for two hours. Subsequently, to the composition were added 0.6 part of polymethylhydrogensiloxane oil (component d) having the formula:

and 29 ppm of chloroplatinic acid (component e) and homogenously dispersed therein on a two-roll rubber mill.

Under the same conditions as in Example I], the composition was pressed into a 2 mm thick sheet and heated at 200 C for 4 hours. The product was found to have the following elastomeric properties:

Hardnes 65 Elongation (9b) 410 Tensile strength (kglcm) 80 Tear strength (kglcm) 34 EXAMPLE V An experiment similar to the one described in Example IV was conducted in which the only difference was that the quantity of polyrnethylhydrogensiloxane oil (component d) was 0.3 part instead of 06 part. The vulcanizate obtained failed to give sufficient elasticity and strength. When L8 parts of polymethylhydrogensiloxane oil (component d) was emloyed, the vulcanizate had the following result, illustrating that when the quantity of vinyl groups present in the components is excessive, condensation takes place among them, preventing suflicient formation of the cross-linked structure:

Hardness 7 5 Elongation k) 220 Tensile strength (kg/cm) 76 Tear strength (kg/cm) l7 While specific embodiments of the invention have been shown and described in detail to illustrate the application of the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such principles.

What is claimed is:

l. A heat-curable elastomeric composition comprising:

a. from about 70 to about 995 parts by weight of a polydiorganosiloxane containing from about 0.02 to about 0.2 mole percent of vinyl group-containing siloxane units and having the general formula R SiO(R,SiO),,,SiR, wherein R is a monovalent hydrocarbyl group and m is at least about 3,000;

. from about 0.5 to about 30 parts by weight of a polydiorganosiloxane containing at least about 2 mole percent of vinyl group-containing siloxane units and having the general formula R' SiO(R,SiO),,SiR' wherein R is a monovalent hydrocarbyl group and n is at least about c. from about 20 to about 200 parts by weight of a silica filler having a surface area of at least about mlg;

a polyorganohydrogensiloxane in an amount such that its Si-H groups are from about 80 to about 250 mole percent relative to the total vinyl groups contained in components (a) and (b) and which have at least three Si-H groups in each molecule; and

e. a platinum compound.

2. The heat-curable elastomeric silicone composition of claim I in which the polyorganosiloxane having at least three Si-H groups in each molecule is selected from the group consisting of trialkylsilyl end-blocked polymethylhydrogensiloxane, a tetrasiloxane having the formula Si {OSi(CH ,),H 1 a siloxane copolymer consisting of SiO: units and (CHQJ'IS i-O-SiH(Cl*l units. and copolymer of methylhydrogensiloxane and dialkylsiloxane.

3. The heat-curable elastomeric silicone composition of claim 1 in which the platinum compound is present in an amount from about 0.5 to about 20 ppm by weight relative to the total polysiloxane components.

4. The heat-curable elastomeric silicone composition of claim I in which the platinum compound is a soluble platinum compound selected from the group consisting of chloroplatinic acid and complexes of platinum chloride and olefines.

5. The heat-curable elastomeric silicone composition of claim 1 containing in addition a dispersing agent selected from the group consisting of low molecular weight siloxane esters and silanols.

6. A process for preparing vulcanizates of elastomeric silioone compositions having exceptional tear strengths comprismg:

a. heating the composition of Claim 1 at about l00 C to about 400 C for from about 30 seconds to about I hour; and

b. post-curing the composition by heating at about 150 C to about 250 C for a period of from about 2 to about 24 hours.

7. The process of claim 6 wherein heating step (a) is carried out under atmospheric pressure.

8. The process of claim 6 wherein heating step (a) is carried out under superatrnospheric pressure.

t I ns t 

2. The heat-curable elastomeric silicone composition of claim 1 in which the polyorganosiloxane having at least three Si-H groups in each molecule is selected from the group consisting of trialkylsilyl end-blocked polymethylhydrogensiloxane, a tetrasiloxane having the formula Si (-O-Si(CH3)2H)4, a siloxane copolymer consisting of SiO2 units and (CH3)2HSi-O-SiH(CH3)2 units, and copolymer of methylhydrogensiloxane and dialkylsiloxane.
 3. The heat-curable elastomeric silicone composition of claim 1 in which the platinum compound is present in an amount from about 0.5 to about 20 ppm by weight relative to the total polysiloxane components.
 4. The heat-curable elastomeric silicone compositIon of claim 1 in which the platinum compound is a soluble platinum compound selected from the group consisting of chloroplatinic acid and complexes of platinum chloride and olefines.
 5. The heat-curable elastomeric silicone composition of claim 1 containing in addition a dispersing agent selected from the group consisting of low molecular weight siloxane esters and silanols.
 6. A process for preparing vulcanizates of elastomeric silicone compositions having exceptional tear strengths comprising: a. heating the composition of Claim 1 at about 100* C to about 400* C for from about 30 seconds to about 1 hour; and b. post-curing the composition by heating at about 150* C to about 250* C for a period of from about 2 to about 24 hours.
 7. The process of claim 6 wherein heating step (a) is carried out under atmospheric pressure.
 8. The process of claim 6 wherein heating step (a) is carried out under superatmospheric pressure. 